1. Field of the Invention
The invention relates to light-transmissive viewing screens, and in particular to lenticular screens having light-absorbing material disposed in grooves between the lenticules of the screen.
2. Description of the Prior Art
Light-transmissive viewing screens such as rear projection screens are commonly provided with integral lenticules or lens elements. These lens elements are shaped to collect image-forming light rays projected onto the rear side of the screen by one or more projection tubes, and to concentrate the rays to form a bright image at the peaks of lens elements on the front or viewing side of the screen.
It is well known that providing a coating of masking material in grooves between the light-emitting lens elements, to absorb ambient light, enhances image contrast. Examples of lenticular screens with such masking material are described in commonly-owned U.S. Pat. No. 4,605,283 to Douglas A. Stanton, which is hereby incorporated by reference. The Stanton patent recognizes the desirability of minimizing contact of the masking material with the screen surfaces defining the grooves. The masking material not only absorbs ambient light incident to the masked grooves at the viewing side of the screen, but also partially absorbs the image-forming light entering the lens elements from the rear of the screen, thereby attenuating the image-forming light eventually reaching the light-emitting peaks of the lens elements. This attenuation occurs wherever the masking material contacts the surface of a lens element, thereby locally increasing the critical angle for total internal reflection (TIR) of light rays striking the rear of the lens elements. As is explained in the Stanton patent, this increase of the critical angle decreases the range of angles from which image-forming light rays received at the rear of the screen will be totally reflected toward the light-emitting peaks of the respective lens elements. To minimize the total area of each lens element contacted by the masking material, thus minimizing the total surface area of the screen for which the TIR is reduced, the Stanton patent proposes that the masking material be provided in the form of a multiplicity of small light-absorbing particles. Each particle makes minimal contact (e.g. point contact) with the outer surface of the lens element against which it is disposed, and the particles are contained within each groove by a layer extending between respective sidewalls of the lens elements which define the groove.
Selective deposition of the light-absorbing masking particles into the grooves of a lenticular screen such as that disclosed in the Stanton patent can be achieved by reasonably simple methods. Any particles which land on the rounded peaks of the disclosed lens elements can be wiped or jarred off and tend to drop into the grooves. However, selective deposition is more difficult with screens having peaks with concave central portions, such as are described in U.S. Pat. No. 4,573,764 to Ralph H. Bradley, which is hereby incorporated by reference. With such screens it has been found difficult to keep the deposited particles out of the concave portions of the peaks. It has also been found difficult to achieve uniform filling of the grooves with the particles. Both of these objectives must be achieved in order to ensure high brightness and high contrast of images formed on the screen.
Another problem experienced is associated with retaining the particles in the grooves without damaging the screen. In the Stanton screen masking arrangement, the layer containing the particles in each groove is preferably formed by heating the face of the screen until the uppermost particles in each groove fuse together. There is only a small margin of error between applying sufficient heat energy to fuse the uppermost particles into a layer and overheating the screen material (typically a plastic material such as polymethyl methacrylate) and causing optical distortion of the lenticules.